A 0-day exploit refers to a cyber attack that takes advantage of a previously unknown vulnerability in a computer application, network, or hardware. The term "0-day" signifies that developers have zero days to fix the vulnerability or issue a patch before it is exploited. These exploits are particularly dangerous because they can be used to gain unauthorized access to systems, allowing attackers to execute malicious code, steal sensitive data, or disrupt services without the software developers having any chance to defend against the attack.